We will investigate the mechanisms by which various T4 phage stuctures function to fulfill their biological roles. In particular we will look into the tail fiber/lipopolysaccharide attachment, the tail fiber/baseplate interaction, and the role of gene 2 product in morphogenesis, DNA ejection, baseplate expansion and protection from restriction by exonuclease V in the cell. We will relate these phenomena to host range extension and speciation. To accomplish these goals we will select new types of phage mutants and compare their altered structures and related functions to each other and to the wild type phage.